Nearly all online activity, including email, internet use, social media, and conducting business, is delivered by way of millions of data centers around the world, ranging from small closet-sized data centers with a single rack of servers up to extremely large data centers with thousands of rack mounted servers, routers, telecommunications, and mass-storage devices. This explosive growth in data center dependence results in large amounts of energy usage. For example, a typical rack of servers may use a few kilowatts of electricity while numerous racks of servers housed in a large data center may use on the order of hundreds of megawatts of electricity over the same period of time not to mention the electricity used by other devices and support resources. Large data centers, in particular, may use more than 100 times the electricity of that used by a typical office building. As a result, electrical power consumption is among the primary costs of operating a data center. For very large data centers, power costs may account for well over 10% of the total operating cost of the data center with the cost continuing to rise with increasing server size. Data-center operators, owners and customers continue to seek ways to reduce data center power consumption and thereby lower operating costs.